The present disclosure relates to a controller for an internal combustion engine that includes a catalyst in an exhaust passage, to which exhaust gas is discharged from a plurality of cylinders. The catalyst purifies the exhaust gas.
Japanese Laid-Open Patent Publication No. 2012-57492 describes a controller for an internal combustion engine that executes air-fuel ratio feedback control. Air-fuel ratio feedback control is executed so that an air-fuel ratio detection value, which is calculated based on an output signal of an air-fuel ratio sensor arranged in the exhaust passage of the internal combustion engine, approaches a target air-fuel ratio. At this time, an air-fuel ratio feedback correction value is updated to decrease the deviation of the air-fuel ratio detection value from the target air-fuel ratio. The air-fuel feedback correction value is used to correct the amount of fuel supplied to each cylinder so that the air-fuel ratio detection value approaches the target air-fuel ratio.
The device described in this publication also executes perturbation control (dither control) to increase the temperature of a three-way catalyst arranged in the exhaust passage. In dither control, some of the cylinders are set to rich combustion cylinders, in which the mixture has an air-fuel ratio that is richer than the stoichiometric air-fuel ratio. The remaining cylinders are set to lean combustion cylinders, in which the mixture has an air-fuel ratio that is leaner than the stoichiometric air-fuel ratio. Additionally, the amount of fuel supplied to each cylinder is adjusted so that an average value of the air-fuel ratios of mixtures in all of the cylinders including the rich combustion cylinders and the lean combustion cylinders equals the target air-fuel ratio. When dither control is executed, unburned fuel components and incomplete combustion components contained in the exhaust gas discharged from the rich combustion cylinders are oxidized in the exhaust passage by oxygen contained in the exhaust gas discharged from the lean combustion cylinders. This increases the temperature of the three-way catalyst.
When air-fuel ratio feedback control is executed, the air-fuel ratio feedback correction value needs to be prevented from having an excessive absolute value. In this regard, air-fuel ratio learning control may be executed to obtain an air-fuel ratio learning value based on the result of air-fuel ratio feedback control. In a controller that executes air-fuel ratio learning control in addition to air-fuel ratio feedback control, the air-fuel ratio learning value is used in addition to the air-fuel ratio feedback correction value. Thus, while limiting the increase in the absolute value of the air-fuel ratio feedback correction value, the amount of fuel supplied to each cylinder is corrected so that the air-fuel ratio detection value approaches the target air-fuel ratio.
During execution of dither control, the lean combustion cylinders discharge exhaust gas containing a large amount of oxygen to the exhaust passage. The rich combustion cylinders discharge exhaust gas containing a small amount of oxygen to the exhaust passage. Thus, during dither control, the exhaust gas passing through the air-fuel ratio sensor in the exhaust passage may contain a large amount of oxygen or a small amount of oxygen. This causes the air-fuel ratio detection value to easily vary and may lower the accuracy of an update of the air-fuel ratio learning value performed by air-fuel ratio learning control. If an air-fuel ratio learning value obtained with low accuracy is used to control the air-fuel ratio, the controllability may be lowered.